ROHM Co., Ltd., has recently announced the development and mass production of a double-diffusion metal-oxide-semiconductor field-effect transistor (DMOSFET) fabricated using silicon carbide (SiC), a material that is expected to see widespread use in next-generation power devices due to its low loss and high breakdown voltage. Proprietary process technologies and screening methods were used to successfully resolve a number of problems associated with the mass production of SiC transistors, including reliability issues caused by SiC crystal defects and variations in characteristics resulting from the high-temperature manufacturing process. This new product can be used in a broad array of applications, including inverters and converters that provide power conversion in products such as air conditioners, solar cells, and industrial equipment.

Complementing the recent focus in the power electronics field on eliminating power consumption in semiconductor devices during power conversion are increasing environmental considerations that have led to a surge of research and development in power devices utilizing SiC, which offers material characteristics superior to those of silicon. Anticipating this trend, ROHM embarked on a pioneering R&D program for SiC devices and modules, developing an SiC MOSFET prototype in 2004, followed by sample power modules exclusively using SiC devices as switching and rectification elements. After successfully mass-producing Japan's first SiC Schottky barrier diode (SBD) in April 2010, the company moved to expand its lineup with higher current rate products. In addition, ROHM acquired SiCrystal AG in Germany to ensure stable supply of high-quality SiC wafers.

ROHM's new DMOSFET delivers a breakdown voltage of 600V and an ON-resistance of 0.4Ω - 10 times lower than equivalent silicon DMOSFETs. Switching times are also reduced by more than 5 times compared with low-ON-resistance silicon IGBTs, providing a combination of high speed performance and low ON-resistance impossible to achieve with conventional silicon devices. The result is dramatically lower loss when used in inverters, converters, or similar devices, along with more compact peripheral components in higher-frequency designs, reducing the required mounting area while lowering the cost of peripheral components. Furthermore, the increase in resistance during high-temperature operation is much lower than that of silicon transistors, offering a significant advantage in the form of low conduction loss during high-output operation. Used in combination with currently available SiC-SBDs in power supply circuits, this new product is expected to contribute to the development of smaller, lower-power systems.

Mass production of SiC transistors has proved particularly challenging to manufacturers worldwide. ROHM was able to overcome several significant obstacles and successfully establish the industry's first mass production system for SiC transistors by developing a proprietary field-weakening architecture and unique screening methods to ensure reliability, as well as original technology that limits the degradation in characteristics caused by the high-temperature processes required in SiC fabrication (up to 1,700ºC).

■Key Features

Compact size and low ON resistance

Minimal increase in ON resistance during high-temperature operation

High-speed switching

High breakdown voltage

■Si vs. SiC MOSFETs

■Terminology

Silicon Carbide(SiC) A compound semiconductor with exceptional properties, including approximately 3 times the bandgap, 10 times the electric field breakdown strength, and 3 times the thermal conductivity of silicon. These characteristics make it well-suited for power device applications and high-temperature operation.

Double-Diffusion Metal-Oxide-Semiconductor Field-Effect Transistor(DMOSFET) A power device structure ideal for use in switching devices requiring high current and breakdown voltage.

Super Junction MOSFET A power MOSFET that delivers low-loss performance compared to conventional designs through the use of an expanded 3D depletion layer.

Insulated Gate Bipolar Transistor(IGBT) A power transistor that provides low ON-resistance by allowing the flow of current by not only electrons, but also holes. However, IGBTs suffer from large switching losses and are incapable of high-speed operation due to the injected hole storage time.

Schottky Barrier Diode(SBD) A diode designed to take advantage of the rectifying (diode-like) properties of a Schottky barrier consisting of a metal-semiconductor junction. SBDs do not suffer from the minority carrier storage effect and excel at high-speed switching.

ON Resistance The electrical resistance exhibited by a power element during its operation. ON-resistance is the most important parameter in determining the performance of a power MOSFET. Smaller values equate with higher performance.

Power Electronics Technology developed for converting power to a desired state and controlling its characteristics using semiconductor devices (electronics). An essential part of modern life, power electronics technologies are used in systems ranging from household electric products to industrial, railroad, power, and other systems. (Excerpt from Power Electronics by Eisuke Shoda and Kazuyuki Kusumoto, published by Ohmsha)